Antibodies against cell-type specific antigens on the surface of mammary epithelial cells (both in humans and in mice) are used for the development of diagnostic procedures for the early and specific detection of breast cancer. These mammary epithelial antigens are shown, by indirect immunofluorescence, to be present on breast cells both in vivo and in vitro, irrespective of their origins (normal or neoplastic breast tissue). They are not detected on cells from other tissues. By quantitative techniques that we have developed to measure these antigens on a cell per cell basis using the flow cytofluorometer, we have shown that neoplastic breast epithelial cells contain levels of mammary epithelial antigens per unit DNA several fold less than levels in normal breast cells. We have also developed very sensitive radioimmunoassay techniques, with which we have established that mammary epithelial antigens are released into circulation of mammary tumor-bearing mice, but not in normal mice. This finding sets the basis for the development of mass screening methods for early detection of breast cancer, since the radioimmunoassay is a simple, inexpensive and highly sensitive procedure. Also, methods are being developed for the localization of breast cancer metastases by whole body scintiscanning techniques using radioactively labeled antibodies against the breast epithelial antigens.